The general arrangement of a vertical seismic profile (VSP) survey is shown in FIG. 1. A tool 10, typically comprising and array of seismic receivers (e.g. geophones) 12, is positioned in a borehole 14 by means of a logging cable 16 connected to surface equipment 18. One or more seismic sources 20 (e.g. airguns) are positioned at the surface some distance from the borehole 14. When the source(s) is fired, seismic waves S travel through the formations 22 surrounding the borehole 14 and are reflected in part from changes in acoustic impedance in the formations due to the presence of bed boundaries 24, and are detected by the receivers 12 in the borehole 14. The signals recorded from the receivers 12 can be interpreted by use of a suitable geophysical model to characterise the formations 22, especially the shape and location of the boundary 24. Often the VSP survey is designed with the intention of investigating a particular area or target within a roughly known boundary. Variations of such surveys can include reverse VSP (sources in borehole, receivers at the surface), walkaway VSP (measurements made from a series of source firing as it is moved progressively further from the borehole), 3D VSP (use of a 2D array of sources at the surface), and drill bit seismic (drill bit as source of signals, receivers at the surface). Tools and techniques for use in VSP surveys are generally well known in the oil and gas industry.
In designing a seismic survey, attention must be paid to the particular regime in which the survey is to be conducted. For a typical surface seismic survey (both sources and receivers located at the surface), reflection wave propagation is essentially symmetric from the source to reflecting boundary and back to the receiver. The symmetry is modified by aspects such as the structure of the underground formations but this can be take into account by using a model which can anticipate such changes. Also, P-S conversion in the waves can result in asymmetry. Borehole seismic surveys such as VSPs are by their very nature asymmetric. Therefore, the problem of designing them is more complex. Since the object of a VSP is often to characterise a region of a boundary (a “target”), especially if the object is to image this target, the positioning of the tool in the borehole and the positioning of the source(s) at the surface is important if the appropriate reflected signals are to be recorded in the borehole. However, it is often the case that it is not possible to have a completely free hand when it comes to positioning the source and receivers. For example, surface geographical features (rivers, lakes, cliffs, gullies, etc.) can limit the ability to place the sources. Also, aspects of the borehole (direction, casing, production and completion equipment) can limit the ability to position the tool in the borehole. Finally, financial and time considerations can also apply limiting the number of sources available or the ability to make measurements with the tool at a number of locations in the borehole.
A typical job design process involves identifying the possible positions for the particular number of sources and receivers available for that job, and using a geophysical model of the underground formations to trace ray paths from the source(s) to the receivers. The positions of the sources and receivers are then adjusted until the model shows that the detected signals arrive from the target. The adjustment of the positions is performed by the user who uses experience and available information to estimate the effect of changing the positions before testing using the model. This makes the whole design process highly dependent on the skill and experience of the user.
It is an object of the present invention to provide a method suitable for designing geophysical surveys and in particular for identifying suitable positions for sources and receivers.